Siemens LME7 Series Technical Instructions page 131

LME Series
Honeywell Modutrol IV with the LME75 (continued)
Sequence of Operation with Valve Proving
1. The LME75 is in standby. All valves are closed and all relays are de-energized. All relay
contacts are as shown in the electrical schematic. The Honeywell Modutrol IV motor
(hereafter referred to as mod motor) is at the low fire position.
2. The LME75 receives a call for heat. Output X2-09.1 is energized in phase 21 to drive the
mod motor to low fire. Relay CR-3 is energized. Output X6-03.3 energizes as well, but
this has no effect since normally closed contact CR-3 is open. Since the mod motor is
already at the low fire position, the low fire proving switch is closed, and actuator
feedback input X2-09.4 is energized.
3. Output X2-09.3 is energized in phase 24 to drive the mod motor to high fire. Relay CR-1
is energized. Normally open contact CR-1A closes, connecting terminals R and B on
0-135 Ohm mod motors or terminals F and - on 4-20 mA mod motors. This drives the
mod motor to the high fire position. Once the mod motor reaches the high fire position,
the high fire proving switch closes, and actuator feedback input X2-09.4 is energized.
Normally open contact CR-1B also closes, but this has no effect since main valve output
terminal X7-04.4 is de-energized during this phase.
4. The LME75 progresses to phase 22 and phase 30 (prepurge). During this time, output
X2-09.3 remains energized*, keeping the mod motor at the high fire position. Actuator
feedback input X2-09.4 is required to remain energized during these phases, proving
that the mod motor is at the high fire position for the full length of the prepurge.
* When valve proving is activated, the valve proving sequence typically takes place
during shutdown. However, after a lockout or a power cycle of the LME75, valve
proving takes place during prepurge as well as postpurge on the next startup. During
the valve proving sequence, output X2-09.3 de-energizes. To ensure the mod motor
remains at the high fire position during prepurge, power to relay CR-1 is latched in
through relay contact CR-1C. The latching circuit is broken when relay CR-3 is energized
in phase 36, thus opening normally closed contact CR-3. Also during valve proving, main
valve outputs X7-04.4 and X7-02.3 are briefly energized, but not at the same time. This
ensures relay CR-2 is never energized during valve proving, which would drive the
actuator off of the high fire position.
SCC Inc. Page 9
Technical Instructions
Document No. LME-1000
Appendix A